Start-up circuit to discharge EMI filter for power saving of power supplies

ABSTRACT

A start-up circuit to discharge EMI filter is developed for power saving. It includes a detection circuit detecting a power source for generating a sample signal. A sample circuit is coupled to the detection circuit for generating a reset signal in response to the sample signal. The reset signal is utilized for discharging a stored voltage of the EMI filter.

REFERENCE TO RELATED APPLICATION

This Application is a continuation application of prior U.S. application Ser. No. 13/870,177 filed on Apr. 25, 2013 which was a Continuation Application of U.S. application Ser. No. 12/539,722, filed 12 Aug. 2009, now U.S. Pat. No. 8,461,915, which claims priority to U.S. provisional application no. 61/217,709, all of which are hereby incorporated herein by reference, and priority thereto is hereby claimed.

BACKGROUND OF THE INVENTION

Field of Invention

The present invention relates to a power supply, and more particularly, the present invention relates to a start-up circuit.

Description of Related Art

Switching mode power supplies have been widely used to provide regulated power for computers, home appliances, communication equipments, etc. In recent years, the problem of power saving in switching mode power supplies has drawn much attention. Based on the restriction of environmental pollution, the computer and other equipment manufactures have been striving to meet the power management and energy conservation requirements.

FIG. 1 shows a traditional approach for filtering electromagnetic interference (EMI) and providing a DC voltage. An EMI filter located between a power source V_(AC) and a bridge rectifier 10 includes a choke L₁, X-capacitors C₁ and C₂. The X-capacitor C₁ is placed across the power source V_(AC). The choke L₁ is coupled between the power source V_(AC) and the bridge rectifier 10. The X-capacitor C₂ is coupled between the choke L₁ and an input of the bridge rectifier 10. A bulk capacitor C_(IN) connected from an output of the bridge rectifier 10 to a ground is for stabilizing the DC voltage V_(BUS) at the output of the bridge rectifier 10. For safety regulations in US and European, a bleeding resistor R_(D) is generally placed across the X-capacitors C₁ or C₂ of the EMI filter. The bleeding resistor R_(D) will discharge the stored energy at the X-capacitors C₁ and C₂ to prevent an electric shock when end-user cut off the power source V_(AC). In fact, the bleeding resistor R_(D) always has a fixed power-loss as long as the X-capacitors C₁ and C₂ have the stored voltage. Besides, for higher power source, the bleeding resistor R_(D) consumes much standby-power when the power supply is operated at no-load. Therefore, the disadvantage of the traditional approach causes a poor power saving at light-load and no-load. Because of the existence of the X-capacitors, it has become a major concern to reduce the standby-power.

SUMMARY OF THE INVENTION

The start-up circuit to discharge EMI filter is for power saving of power supplies according to the present invention. It includes a detection circuit detecting a power source for generating a sample signal. A sample circuit is coupled to the detection circuit for generating a reset signal in response to the sample signal. The reset signal is for discharging a stored voltage of the EMI filter. The start-up circuit further includes a delay circuit coupled to the sample circuit for generating a discharging signal in response to the reset signal. The discharging signal is coupled to drive the detection circuit for discharging the stored voltage of an X-capacitor of the EMI filter when the sample signal is still larger than the reference signal over a period.

BRIEF DESCRIPTION OF ACCOMPANIED DRAWINGS

The accompanying drawings are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the present invention. In the drawings,

FIG. 1 shows a circuit diagram of a traditional EMI filter.

FIG. 2 shows a circuit diagram of a preferred embodiment of a start-up circuit according to the present invention.

FIG. 3 shows the waveform of the power source and the high-voltage signal according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2 is a preferred embodiment of a start-up circuit according to the present invention. The start-up circuit is utilized to discharge the EMI filter for power saving of power supplies. The EMI filter includes the choke L₁, X-capacitors C₁ and C₂, the bulk capacitor C_(1N), and the bridge rectifier 10 for filtering EMI and providing the DC voltage V_(BUS). The start-up circuit includes a rectifier, a series resistor R₁, a detection circuit 20, a sample circuit 30 and a delay circuit 40. The rectifier can be a full-wave rectifier having a first diode D₁ and a second diode D₂ according to one embodiment of the present invention, Anodes of the first diode D₁ and the second diode D₂ are connected to the power source V_(AC) respectively. Cathodes of the first diode D₁ and the second diode D₂ are together connected to one terminal of the series resistor R₁. The other terminal of the series resistor R₁ generates a high-voltage signal V_(HV) through the full-wave rectification of the first diode D₁ and the second diode D₂. Thus, the rectifier is coupled to the power source V_(AC) for rectifying the power source V_(AC) to generate the high-voltage signal V_(HV).

The detection circuit 20 is coupled to the series resistor R₁ for detecting the high-voltage signal V_(HV) to generate a sample signal V_(SP) and a supply voltage V_(DD). Therefore, the detection circuit 20 detects the power source V_(AC) for generating the sample signal V_(SP) through detecting the high-voltage signal V_(HV). The sample signal V_(SP) is correlated to the high-voltage signal V_(HV). The sample circuit 30 is coupled to the detection circuit 20 for generating a reset signal V_(RESET) in response to a clock signal V_(CLK) and the sample signal V_(SP). The delay circuit 40 is coupled to the sample circuit 30 for generating a discharging signal V_(DIS) and a power-on signal V_(ON) in response to a pulse signal V_(PULSE) and the reset signal V_(RESET). The detection circuit 20 is coupled to the X-capacitors C₁ and C₂ of the EMI filter and receives the discharging signal V_(DIS) for pulling down the supply voltage V_(DD) and discharging the stored voltage of the X-capacitors C₁ and C₂. The power-on signal V_(ON) is used for turning on a PWM circuit to regulate the output of the power supply. The PWM circuit is a prior-art technique, so here is no detailed description about it.

The detection circuit 20 includes a high-voltage switch J₁, a first transistor M₁, a switch circuit having a second transistor M₂ and a third transistor M₃, a fourth transistor M₄, a discharge resistor R₂ and a hysteresis comparator 210. The high-voltage switch J₁ formed by a Junction Field Effect Transistor (JFET) has a drain terminal coupled to the series resistor R₁ for receiving the high-voltage signal V_(HV). The drain terminal of the high-voltage switch J₁ is further coupled to the X-capacitors C₁ and C₂ through the series resistor R₁, the first diode D₁ and the second diode D₂. The first transistor M₁ has a drain terminal coupled to a source terminal of the high-voltage switch J₁, a gate terminal coupled to a gate terminal of the high-voltage switch J₁. The sample signal V_(SP) is generated at the source terminal of the high-voltage switch J₁ and the drain terminal of the first transistor M₁. A trigger signal V_(GJ1) is generated at the gate terminals of the high-voltage switch J₁ and the first transistor M₁. The second transistor M₂ has a drain terminal coupled to the gate terminals of the high-voltage switch J₁ and the first transistor M₁, a source terminal coupled to the source terminal of the high-voltage switch J₁ and the drain terminal of the first transistor M₁ for receiving the sample signal V_(SP). The third transistor M₃ has a drain terminal coupled to the drain terminal of the second transistor M₂ for receiving the trigger signal V_(GJ1), a source terminal that is coupled to a ground, a gate terminal coupled to a gate terminal of the second transistor M₂.

The fourth transistor M₄ has a drain terminal coupled to a source terminal of the first transistor M₁, a source terminal coupled to one terminal of the discharge resistor R₂. The other terminal of the discharge resistor R₂ is coupled to the ground. A positive input of the hysteresis comparator 210 is coupled to the source terminal of the first transistor M₁ and the drain terminal of the fourth transistor M₄ for receiving the supply voltage V_(DD). The hysteresis comparator 210 has a negative input to receive a threshold signal V_(TH). An output of the hysteresis comparator 210 generates a switching signal V_(SW) that is coupled to the gate terminals of the second transistor M₂ and the third transistor M₃. By comparing the supply voltage V_(DD) with the threshold signal V_(TH), the switching signal V_(SW) is generated and controls an on/off status of the second transistor M₂ and the third transistor M₃. The hysteresis comparator 210 is only one embodiment of the present invention, and the prevent invention isn't limited to the hysteresis comparator 210. In this manner, the switching signal V_(SW) is at a high-level once the supply voltage V_(DD) is larger than an upper-limit of the threshold signal V_(TH). On the contrary, the switching signal V_(SW) is at a low-level once the supply voltage V_(DD) is smaller than a lower-limit of the threshold signal V_(TH). The lower-limit of the threshold signal V_(TH) is also called under voltage lockout (UVLO). Because of the hysteresis characteristic of the hysteresis comparator 210, the difference between the upper-limit and the lower-limit always keeps a fixed voltage range.

The sample circuit 30 includes a fifth transistor M₅, a pull-down resistor R₃, a voltage comparator 310 and a NAND gate 320. The fifth transistor M₅ has a drain terminal coupled to the detection circuit 20 for receiving the sample signal V_(SP), a source terminal coupled to one terminal of the pull-down resistor R₃ for generating an input signal V_(INAC). The other terminal of the pull-down resistor R₃ is coupled to the ground. The voltage comparator 310 has a positive input receiving a reference signal V_(REF), a negative input coupled to the source terminal of the fifth transistor M₅ for receiving the input signal V_(INAC). The input signal V_(INAC) is proportional to the high-voltage signal V_(HV) and correlated to the sample signal V_(SP) once the high-voltage switch J₁ and the fifth transistor M₅ are turned on. A first input of the NAND gate 320 coupled to a gate terminal of the fifth transistor M₅ receives the clock signal V_(CLK). A period of the clock signal V_(CLK) is T₁. A second input of the NAND gate 320 coupled to an output of the voltage comparator 310 receives a first signal V₁. The first signal V₁ is generated by comparing the input signal V_(INAC) with the reference signal V_(REF). The output of the NAND gate 320 generates the reset signal V_(RESET). As mentioned above, the voltage comparator 310 is utilized for generating the reset signal V_(RESET) in response to the sample signal V_(SP) and the reference signal V_(REF).

The delay circuit 40 includes a first flip-flop 410 and a second flip-flop 420. The first flip-flop 410 has an input D receiving the supply voltage V_(DD), a clock-input CK receiving the pulse signal V_(PULSE), a reset-input R receiving the reset signal V_(RESET). A period of the pulse signal V_(PULSE) is T₂. The period T₂ is much larger than the period T₁ about 20 times. The second flip-flop 420 has an input D receiving the supply voltage V_(DD), a clock-input CK coupled to an output Q of the first flip-flop 410, a reset-input R receiving the reset signal V_(RESET), an output Q generating the discharging signal V_(DIS) coupled to a gate terminal of the fourth transistor M₄, an output QN generating the power-on signal V_(ON) coupled to the PWM circuit to turn on the PWM circuit to regulate the output of the power supply.

FIG. 3 shows the waveform of the power source and the high-voltage signal. The period of the power source V_(AC) is about 20 ms if the input supply frequency is 50 Hz. The high-voltage signal V_(HV) is generated through the full-wave rectification of the first diode D₁ and the second diode D₂. The clock signal V_(CLK) is used to sample the high-voltage signal V_(HV) for each period T₁. If the power source V_(AC) is shut down in the peak value of the negative half-wave of the power source V_(AC), the amplitude of the high-voltage signal V_(HV) will last a high DC voltage for a long time. According to the present invention, the delay circuit 40 will count a period to be period T₃ and turned off the PWM circuit when the amplitude of the sample signal V_(SP) is still larger than the reference signal V F over the period T₃. It means that the delay circuit 40 will turn off the PWM circuit when the high-voltage signal V_(HV) is still larger than the reference signal V_(REF) over the period T₃. In the meantime, the X-capacitors C₁ and C₂ of the EMI filter is discharged and the supply voltage V_(DD) is pulled to UVLO. The period T₃ is equal to the period T₂ or higher than the period T₂.

Referring to the detection circuit 20 of FIG. 2, when the power source V_(AC) is switched on, the drain terminal of the high-voltage switch J₁ receiving the high-voltage V_(HV) is turned on immediately. The switching signal V_(SW) is at a low-level since the supply voltage V_(DD) doesn't be created yet. At this time, the third transistor M₃ is turned off and the second transistor M₂ is turned on. The sample signal V_(SP) is about a threshold voltage of the second transistor M₂ and generated at the source terminal of the high-voltage switch J₁ and the drain terminal of the first transistor M₁. Because the second transistor M₂ is turned on, the trigger signal V_(GJ1) is the same as the sample signal V_(SP) and generated at the gate terminals of the high-voltage switch J₁ and the first transistor M₁. In the meantime, the first transistor M₁ is turned on and the supply voltage V_(DD) is charged by the high-voltage signal V_(HV). The first transistor M₁ serves as a charge transistor for charging the supply voltage V_(DD). When the supply voltage V_(DD) reaches to the upper-limit of the threshold signal V_(TD), the switching signal V_(SW) is at a high-level. At this time, the third transistor M₃ is turned on and the second transistor M₂ is turned off. Because the trigger signal V_(GJ1) is pulled-down to the ground, the first transistor M₁ is turned off and the gate terminal of the high-voltage switch J₁ is at a low-level. During a short period, the source-to-gate voltage of the high-voltage switch J₁ will be higher than a threshold and the high-voltage switch J₁ is turned off.

Referring to the sample circuit 30 of FIG. 2, the fifth transistor M₅ is turned on once the clock signal V_(CLK) is at a high-level. Because of the voltage drop in the pull-down resistor R₃, the source-to-gate voltage of the high-voltage switch J₁ will be lower than the threshold and the high-voltage switch J₁ is turned on. On the other hand, the high-voltage switch J₁ is turned off once the clock signal V_(CLK) is at a low-level. The period T₁ of the clock signal V_(CLK) is about 0.6 ms according to one embodiment of the present invention. When the power source V_(CLK) is normal operation and the fifth transistor M₅ is turned on, the input signal V_(INAC) with 120 Hz sinusoidal is proportional to the high-voltage signal V_(HV). The first signal V₁ is generated by comparing the input signal V_(INAC) with the reference signal V_(REF). The first signal V₁ is at a high-level and the reset signal V_(RESET) is generated once the input signal V_(INAC) is smaller than the reference signal V_(REF). At this time, the discharging signal V_(DIS) at the output Q of the second flip-flop 420 is at a low-level and the fourth transistor M₄ is turned off whatever the pulse signal V_(PULSE) is at a high-level or a low-level. The power-on signal V_(ON) at the output QN of the second flip-flop 420 is at a high-level to turn on the PWM circuit. On the contrary, the first signal V₁ is at a low-level and the reset signal V_(RESET) is at a high-level not to reset once the input signal V_(INAC) is larger than the reference signal V_(REF). The delay circuit 40 starts to count when the reset signal V_(RESET) and the pulse signal V_(PULSE) are at the high-level. Since the power source V_(AC) is normal operation and the high-voltage signal V_(HV) is sampled by the clock signal V_(CLK), the input signal V_(INAC) will be smaller than the reference signal V_(REF) again. The discharging signal V_(DIS) is at the low-level to turn off the fourth transistor M₄ and the power-on signal V_(ON) is at the high-level to turn on the PWM circuit.

When the power source V_(AC) is shut down, the high-voltage signal V_(HV) will not be 120 Hz sinusoidal and also will last the high DC voltage. During the shut down moment, the supply voltage V_(DD) keeps a fixed voltage and the switching signal V_(SW) is at the high-level because the high-voltage signal V_(HV) still has the high DC voltage. At this time, by sampling the high-voltage signal V_(HV), the sample signal V_(SP) is still larger than the reference signal V_(REF). Therefore, the input signal V_(INAC) is always larger than the reference signal V_(REF). The delay circuit 40 will count the period T₃ through the pulse signal V_(PULSE). The period T₂ of the pulse signal V_(PULSE) is about 12 ms and the period T₃ is about 24 ms according to one embodiment of the present invention. The power-on signal V_(ON) at the output QN of the second flip-flop 420 will be at a low-level to turn off the PWM circuit after the period T₃.

In the meantime, the discharging signal V_(DIS) of the output Q of the second flip-flop 420 will be at a high-level to turn on the fourth transistor M₄ after the period T₃. Because of the voltage drop in the discharge resistor R₂, the supply voltage V_(DD) will be lower than the lower-limit of the threshold signal V_(TH). Therefore, the delay circuit 40 will turn off the PWM circuit and pull the supply voltage V_(DD) to UVLO. The switching signal V_(SW) is at the low-level and the third transistor M₃ is turned off after the supply voltage V_(DD) is lower than the lower-limit of the threshold signal V_(TH). At this time, the first transistor M₁ and the second transistor M₂ are turned on. The high-voltage switch J₁ is turned on in response to the difference between the source and gate terminals of the high-voltage switch J₁ is at a low-level. The stored voltage at the X-capacitors C₁ and C₂ of the EMI filter will be discharged at the series resistor R₁ and the discharge resistor R₂ through the on-status of the high-voltage switch J₁, the first transistor M₁ and the fourth transistor M₄. Thus, the present invention provides a discharge path to solve the drawback as mentioned above once the supply voltage V_(DD) is lower than the lower-limit of the threshold signal V_(TH).

According to the description above, when the sample signal V_(SP) is still larger than the reference signal V_(REF) over the period T₃, the discharging signal V_(DIS) is at the high-level for driving the detection circuit 20 in response to the reset signal V_(RESET). The discharging signal V_(DIS) drives the detection circuit 20 for discharging the stored voltage of the X-capacitors C₁ and C₂ of the EMI filter and pulling down the supply voltage V_(DD). Therefore, when the sample signal V_(SP) is still larger than the reference signal V_(REF) over the period, the reset signal V_(RESET) is utilized for discharging the stored voltage of the X-capacitors C₁ and C₂ of the EMI filter and pulling down the supply voltage V_(DD). The fourth transistor M₄ of the detection circuit 20 serves as a discharge transistor due to the stored voltage of the X-capacitors C₁ and C₂ is discharged through the fourth transistor M₄.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

What is claimed is:
 1. A discharge circuit of an EMI filter having an X-capacitor comprising: a monitor circuit configured for coupling to the EMI filter for monitoring a power source; a first transistor of the monitor circuit configured for receiving a signal from the power source, the signal having a first waveform correlated to a waveform of the power source; and the monitor circuit configured to detect that the power source is shut down over a period of time and responsively enable the first transistor to provide a discharging path to conduct a discharge current from the EMI filter through the first transistor for discharging a stored voltage of the EMI filter wherein the discharging path is cut off when the power source is in an active state.
 2. The discharge circuit of claim 1 wherein the first transistor is configured to form a sample signal having a second waveform that is representative of the waveform of the power source; and a detection circuit configured for providing the discharging path in response to detecting that the sample signal is larger than a reference signal over the period of time wherein the period of time is no less than a period of a waveform of the power source.
 3. The discharge circuit of claim 2 further including a timer circuit configured to form the period of time wherein the timer circuit is reset in response to the sample signal having a value that is less than the reference signal.
 4. The discharge circuit of claim 1 wherein the monitor circuit also enables the first transistor to conduct a charging current to form an operating voltage for the monitor circuit wherein the charging current is cut off when the first transistor is enabled to provide the discharging path.
 5. The discharge circuit of claim 1 wherein the monitor circuit includes a detection circuit configured to receive a sample signal from the first transistor, and a comparator to compare the sample signal to a reference value.
 6. The discharge circuit of claim 1 wherein the monitor circuit includes a second transistor coupled in series with the first transistor, the monitor circuit configured to enable the second transistor to provide the discharging path to conduct the discharge current through the first transistor and through the second transistor wherein the second transistor is disabled when the power source is in an active state.
 7. The discharge circuit of claim 6 wherein the first transistor is configured to form a supply voltage for the monitor circuit and wherein enabling the second transistor discharges the supply voltage.
 8. The discharge circuit of claim 6 wherein the first transistor is enabled while the power source is in the active state.
 9. The discharge circuit of claim 1 wherein the monitor circuit includes a sample circuit configured to generate a reset signal to reset a timer circuit in response to a sample signal having a value less than a reference value wherein the sample signal is received from the first transistor.
 10. The discharge circuit of claim 9 wherein the sample circuit includes a voltage comparator that compares the sample signal to reference signal and periodically resets the timer circuit in response to the active state of the power source.
 11. The discharge circuit of claim 1 wherein the first transistor is a high-voltage switch configured to receive the signal and form a sample signal that is representative of the signal wherein the monitor circuit also includes: a sample circuit including a comparator coupled to compare the sample signal to a reference signal and cause the sample circuit to reset a timer in response to the active state of the power source; and a discharge transistor coupled to the high-voltage switch, the discharge transistor providing the discharging path to conduct the discharge current from the EMI filter through the high-voltage switch and through the discharge transistor wherein the discharge transistor is disabled in response to detecting the active state of the power source.
 12. A method of configuring a circuit of an EMI filter comprising: configuring a monitoring circuit to monitor an AC power source; and configuring the monitoring circuit to detect the AC power source being shut down for a period of time and to responsively form a discharge path for discharging a stored energy of the EMI filter wherein the discharge path is disabled in response to the monitoring circuit detecting an active state of the AC power source.
 13. The method of claim 12 further including configuring a high-voltage transistor to receive a signal that is representative the AC power source and to form a sample signal that is representative of the AC power source; and coupling a discharge transistor to receive a discharge current from the high-voltage transistor wherein the discharge transistor is enabled to conduct the discharge current in response to the monitoring circuit detecting the AC power source being shut down for the period of time and is disabled otherwise.
 14. The method of claim 13 further including configuring a sample and delay circuit to enable the discharge transistor in response to detecting that the sample signal is greater than a reference signal for the period of time wherein the discharge current is conducted from the EMI filter through the high-voltage transistor and the discharge transistor.
 15. The method of claim 14 including configuring a comparator to compare the sample signal to the reference signal and to reset the delay circuit in response to detecting the sample signal less than the reference signal.
 16. A method of configuring a discharge circuit for an EMI filer comprising: configuring a high-voltage transistor to receive a signal that is representative of an AC power source and form a sample signal that is representative of the signal; coupling a discharge transistor in series with the high-voltage transistor; and configuring a delay circuit to enable the discharge transistor to conduct a discharge current from the EMI filter through the high-voltage transistor and through the discharge transistor in response to detecting that the AC power source is shut down over a period of time.
 17. The method of claim 16 wherein detecting that the AC power source is shut down over the period of time includes configuring the delay circuit to detect the sample signal having a value greater than a reference value over the period of time.
 18. The method of claim 17 further including configuring the delay circuit for being reset in response to the sample signal having a value less than the reference value.
 19. The method of claim 16 including configuring the delay circuit to disable the discharge transistor in response to detecting an active state of the AC power source wherein the high-voltage transistor is enabled during a time of the active state of the AC power source. 